1. Field of the Invention
The present invention relates to an electrical power steering apparatus having a motor which produces a steering assisting torque in accordance with a steering torque of a steering wheel, and in particular to an improved electrical power steering apparatus.
2. Description of the Prior Art
A prior art electrical power steering apparatus is shown, for example, in Japanese Patent Laid-Open Publication No. 59-77966 (1984).
In this prior art apparatus, a steering torque applied to a steering wheel is detected by a steering torque detector, and a steering assisting torque is produced by a motor in accordance with the steering torque detection value. Since the temperature rise of the motor is proportional to the square of a load current, the load current is detected by motor current detection means and the motor is controlled so that the current value does not exceed a predetermined value thereby suppressing the temperature rise.
Furthermore, in such an electrical power steering apparatus, since the motor is driven based on the detection value detected by the steering torque detector, when the steering torque detector is in an abnormal condition and a steering detection value different from the value of the steering torque actually applied to the steering wheel in outputted, the motor will produce an unnecessary assisting torque affecting the driving stability of a vehicle.
For this reason, in the prior art, an abnormality detection device is used to detect whether the output of the steering torque detector is normal or abnormal. This abnormality detection device determines a normal condition and an abnormal condition, and it is determined as being in the normal condition when the output value of the steering torque detector is in a normal range defined by an upper limit value and a lower limit value which are set beforehand, whereas it is determined as the abnormal condition when the output value of the steering torque detector is out of the normal range.
However, in the prior art electrical power steering apparatus, the temperature rise of the motor due to overload is suppressed by detecting a load current of the motor which produces the steering assisting torque. Generally, a relationship between the steering torque applied to the steering wheel and the output voltage of the steering torque detector is represented as shown in FIG. 7. In this relationship, when the steering torque detector is in the normal condition with the steering torque being zero, as shown by the solid line, the output of the steering torque detector is also zero. Thus, the driving current of the motor will be zero to put the motor in a stop driving condition. However, in some cases, due to the influences of a friction or the like in a steering system connected to the steering wheel and/or in the steering torque detector, when the steering torque is zero, the output voltage of the steering torque detector does not become zero as shown by the chain line in FIG. 1. Instead, the steering torque detector exhibits a hysteresis characteristic which is defined as the abnormal condition. When a remaining voltage due to the hysteresis characteristic exceeds a blind band width which is set beforehand near zero steering torque, as shown in FIG. 2 which shows a relationship between the output voltage of the steering torque detector and the motor driving current, the motor will be supplied with a driving current even when the steering wheel is not steered. If this situation is left as it is, there is a possibility that the motor and its control circuit will overheat and burn. However, since the load current of the motor at this time is relatively small and not in the overload condition, it is impossible to detect with the load current detection means. As a result, a problem exists in that the overheating and burning cannot be prevented.
Furthermore, referring to the abnormality detection device of the torque detector in the prior art, since the normal and abnormal conditions of the torque detector are determined merely depending on whether or not the output value of the torque detector is in the normal range defined by the upper limit value and the lower limit value, it is necessary to extend the width of the normal range in order to cope with the non-uniformity of output values due to individual differences of torque detectors. However, when breaking of wire occurs in the applied voltage side of the torque detector, the output value exceeds the upper limit and lower limit values of the normal condition, when short-circuit occurs within the torque detector, a change in the output values will be relatively small. Accordingly, in order to detect both the breaking of wire and short-circuiting, it has been necessary to narrow the range of the output values of the detector, and this in turn requires narrowing the set width of the normal range.
When the torque detector is used in the narrow range of the output values as mentioned above, the anti-noise property is degraded, and at the same time, the mechanical components constituting the torque detector are required to be finished with high accuracy. Thus, another problem arises in that the manufacturing cost in increased.